LED strip 2 digit segment display problem

hi guys, im david bolton from cape town .

i'm currently building a lap counter for my stockcar club.

i got the Arduino UNO programmed to multiplex count on common cathode segment display

but now i want to upscale to led strips but i sit with a problem.

i used a ULN2803 arrqay but then i figured out,, its switching ground only and the led strips all need 12v

so now im stuck with what to do with the commons?

my electronics knowledge isnt 100%

hope you can help me!

can anyone shed some light please?

It's not very clear what you're asking. If you're switching from a smaller segment display to LED strips you should be able to use the same circuit assuming it's capable of the larger amount of current that the LED strips require.

Give us some specs on the LED strip current requirements and show us what you have for a circuit now; we need a schematic (hand drawn if necessary).

i copied this setup

my friend added code for 2 buttons to count up and down - that's all working, circuit works fine like it is... setup as common cathode currently, i tried using a uln2803 but i did not knwo where the common cathode lines must gomust go...

however i need to upscale to 50cm high digits

the 12v led strips are rated 14w per meter. i only want to use about 30cm per segment. so im guessing 1 segment will pull about 250ma

i was thinking of using all 14 pins rather than multiplex?

then i can control each pin with a transistor? ( switch ground)

justcruzin:
i was thinking of using all 14 pins rather than multiplex?

then i can control each pin with a transistor? ( switch ground)

I agree, if this is for outdoor use, you need maximum brightness. That means no multiplexing.

If you are building these displays from strips, you can choose to make them common cathode or common anode.

If you make them common anode it will be easier.

With common anode, you can simply use an ordinary npn transistor for each segment (between the segment -ve and ground). BC337 would be ok for example. The Arduino pins (or shift register pins) can drive the transistor bases directly with 1K resistors to limit the base current. Alternatively use 2 x tpic6a595 high-current shift registers. The "a" variant can sink up to 350mA per pin.

With common cathode, its slightly trickier. You can use ordinary pnp transistors like BC327, between +12V and each segment +ve. But you can't drive the pnp bases directly with Arduino/shift register pins. They would be damaged by the 12V supply. So you would need an npn to pull each pnp base down and 10K to pull the pnp base up to 12V. Again, use 1K to limit the pnp base current and 4K7 to limit the npn base current. Alternatively use 2 x ULN2003/2803/tpic6c595 instead of the npn.

Your estimate of 250mA per segment may be too high. 30cm segments, assuming ordinary 60 leds/metre srtips, would need 60 * 0.3 * 20 / 3 = 120mA. Were you thinking of two parallel strips for each segment?

Paul

will this work?

instead of using npn transistors… can i use the ULN 2803 or other uln 2003?

this guy used transistors and common cathode setup.

pretty neat!!!

sorry, its a common anode setup using the ULN chips - they do hold 500ma per channel i read..

justcruzin:
sorry, its a common anode setup using the ULN chips - they do hold 500ma per channel i read..

The ULN chips can't sustain 500mA continuously if multiple channels are on at the same time.

Please check my earlier post, I updated it to add more detail not realising you would reply so quickly!

i struggle to find tpic6a595 in cape town

i think im going to rather go bc337 - i see they 800ma

but if you look at his design. its pretty straight forward right?

will you be able to draw a quick drawing on how to cconnect that transistor… what i saw on most sites is some of the npn transistors dont have same pin out.

looking directly at the flat part of the bc337 ( as u view the digits) i see on google its says pins:
1-2-3 are C-B-E

correct?

justcruzin:
i struggle to find tpic6a595 in cape town

So buy them on eBay.

justcruzin:
i think im going to rather go bc337 - i see they 800ma
but if you look at his design. its pretty straight forward right?

Looks OK.

justcruzin:
will you be able to draw a quick drawing on how to cconnect that transistor.. what i saw on most sites is some of the npn transistors dont have same pin out.

The connections in that Instructible look OK. Can't believe I said that. They are normally terrible.

justcruzin:
looking directly at the flat part of the bc337 ( as u view the digits) i see on google its says pins:
1-2-3 are C-B-E

correct?

Yes. Here is the data sheet.

this is what confuses me… the pinout… some say 1-2-3 others say 3-2-1 when using the flat section as reference